Conventionally, analog-to-digital conversion (referred to as A/D conversion hereinafter) has been realized through the digitizing, quantization and encoding processings, and these processings have been conventionally performed through the electrical signal processings using semiconductor devices.
Currently, there is a demand for analog-to-digital conversion processing by the high-speed optical processing. For example, non-patent documents 1 and 2 (referred to as prior art examples hereinafter) disclose a method of quantizing an optical analog signal using a Mach-Zehnder interferometer type optical modulator as a sample hold circuit, performing photoelectric conversion using a photo-detector, and encoding a converted electric signal using electric circuits.                Patent Document 1: Japanese patent laid-open publication No. P01-271730A.        Patent Document 2: Japanese patent laid-open publication No. P2000-010129A.        Patent Document 3: Japanese patent laid-open publication No. P9-033967A.        Patent Document 4: Japanese patent laid-open publication No. P9-222620A.        Patent Document 5: Japanese patent laid-open publication No. P9-102991A.        Patent Document 6: Japanese patent laid-open publication No. P2000-321606A.        Patent Document 7: Japanese patent laid-open publication No. P2001-117125A.        Patent Document 8: Japanese patent laid-open publication No. P8-146473A.        Patent Document 9: Japanese patent laid-open publication No. P2002-525647A.        Patent Document 10: Japanese patent laid-open publication No. P2003-107541A.        Non-Patent Document 1: Henry F. Taylor, “An Optical Analog-to-digital Converter-Design and Analysis”, IEEE Journal of Quantum Electronics, Vol. QE-15, No. 4, Apr. 1979.        Non-Patent Document 2: B. Jalali et al., “Optical folding-flash analog-to-digital converter with analog encoding”, Optics Letters, Optical Society of America, Vol. 20, No. 18, Sep. 15, 1995.        Non-Patent Document 3: N.J. Doran et al., “Nonlinear-optical loop mirror”, Optical Letters, Optics Society of America, Vol. 13, No. 1, January 1988.        Non-Patent Document 4: Takashi Yamamoto et al., “Demultiplexing of subterabit TDM signal by using ultrafast nonlinear optical loop mirror”, Technical Report of the Institute of Electronics Information and Communication Engineers, C-I, issued from the Institute of Electronics Information and Communication Engineers, Vol. J82-C-1, pp. 109-116, March 1999.        Non-Patent Document 5: Govind P. Agrawal, “NONLINEAR FIBER OPTICS”, Academic Press, ISBN: 0120451425, 2nd Edition, pp. 210-211, 1995.        Non-Patent Document 6: Stephen M. Jensen, “The Nonlinear Coherent Coupler”, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982.        Non-Patent Document 7: William S. Wong et al., “Self-switching of optical pulses in dispersion-imbalanced nonlinear loop mirrors”, Optics Letters, Optical Society of America, Vol. 22, pp. 1150-1152, 1997.        Non-Patent Document 8: I. Y. Khrushchev et al., “High-quality laser diode pulse compression in dispersion-imbalanced loop mirror”, Electronics Letters, Vol. 34, pp. 1009-1010, May 1998.        Non-Patent Document 9: K. R. Tamura et al., “Spectral-Smoothing and Pedestal Reduction of Wavelength Tunable Quasi-Adiabatically Compressed Femtosecond Solitons Using a Dispersion-Flattened Dispersion-Imbalanced Loop Mirror”, IEEE Photonics Technology Letters, Vol. 11, pp. 230-232, February 1999.        Non-Patent Document 10: K.J. Blow et al., “Demonstration of the nonlinear fiber loop mirror as an ultrafast all-optical demultiplexer”, Electronics Letters, Vol. 26, pp. 962-964, 1990.        